The present invention generally relates to image forming apparatuses, and more particularly to an image forming apparatus which forms an image on a recording medium by electrophotography. The present invention is suited for application to a printer, a copying machine and a facsimile machine which use a laser beam to print the image on the recording medium such as a sheet of paper and an envelope.
Conventionally, in an image forming apparatus of the electrophotography type, a photoconductive or photosensitive body 1 in the form of an endless belt is provided across a driving roller 2 and a roller 3 as shown in FIG.1. The photosensitive body 1 is driven by rotating the driving roller 2, and a charger 4 uniformly charges the surface of the photosensitive body 1. A write part 5 irradiates a laser beam on the charged surface of the photosensitive body 1 to form an electrostatic image, and this electrostatic image is developed by a developer unit 6 which visualizes the electrostatic image into a toner image. A transfer charger 7 transfers the toner image onto a recording medium such as a sheet of paper. Such an image forming apparatus is proposed in a Japanese Laid-Open Utility Model Application No.61-193477, for example.
On the other hand, a Japanese Laid-Open Patent Application No.1-316774 proposes a structure shown in FIG.2. In FIG.2, the transfer roller 9 presses against the photosensitive body 8 which has a drum shape, and the recording medium is inserted and pinched between the transfer roller 9 and the photosensitive body 8 when printing the image on the recording medium. A voltage is applied to the transfer roller 9 when the recording medium is inserted so that the toner image on the surface of the photosensitive body 8 is transferred onto the recording medium, similarly as in the case of the image forming apparatus shown in FIG.1.
However, in the conventional image forming apparatuses described above, problems occur when the recording medium is a thin paper which is weak, that is, has a poor rigidity, along the direction in which the paper is transported within the image forming apparatus. In other words, the paper may curl around photosensitive body 8 or the transfer roller 9 after the toner image is transferred onto the paper instead of being separated from the photosensitive body 8 or the transfer roller 9 to be ejected.
In addition, because the recording medium tends to curl around the photosensitive body 8 and the transfer roller 9 becomes deformed with time after recording mediums of various sizes and thicknesses are inserted between the photosensitive body 8 and the transfer roller 9, the pinching force exerted by the photosensitive body and the transfer roller 9 on the recording medium becomes unstable. For this reason, there is a problem in that the transfer of the toner image onto the recording medium becomes non-uniform, thereby resulting in a poor picture quality of the formed image.